Electrochromic devices are devices that change light (and heat) transmission properties in response to voltage applied across the device. Electrochromic devices can be fabricated which electrically switch between transparent and translucent states (where the transmitted light is colored). Furthermore, certain transition metal hydride electrochromic devices can be fabricated which switch between transparent and reflective states. Electrochromic devices are incorporated in a range of products, including architectural windows, rear-view mirrors, and protective glass for museum display cases. When they are incorporated in architectural windows there is a need for the electrochromic devices to have a guaranteed lifetime of at least ten years and preferably thirty years or more. However, exposure of the electrochromic devices to atmospheric oxygen and water can degrade the performance of the devices and reduce the lifetime of the devices. Therefore, there is a need for electrochromic devices designed to withstand the deleterious effects of ambient oxidants.
Architectural windows are generally in the form of an insulated glass unit (IGU). An IGU comprises two spaced apart panes of glass sealed along all four edges. The interior volume is filled with an inert gas, such as argon, so as to provide thermal insulation. When an electrochromic device is incorporated into the IGU it is fabricated on the exterior glass pane (the outdoors facing pane) and is positioned on the interior facing surface thereof. The inert environment within the IGU does not affect the performance of the electrochromic device. However, in case of a failure of the seal of the IGU, for example, there is still a need to protect the electrochromic devices against ambient oxidants.
A prior art electrochromic device 100 is represented in FIG. 1. See U.S. Pat. No. 5,995,271 to Zieba et al. The device 100 comprises a glass substrate 110, lower transparent conductive oxide (TCO) layer 120, a cathode 130, a solid electrolyte 140, a counter electrode 150, upper TCO layer 160, a protective coating 170, a first electrical contact 180 (to the lower TCO layer 120), and a second electrical contact 190 (to the upper TCO layer 160). Furthermore, there may be a diffusion barrier layer (not shown) between the glass substrate 110 and the lower TCO layer 120, to reduce the diffusion of ions from the glass substrate into the TCO layer, and vice versa. Note that the component layers are not drawn to scale in the electrochromic device shown in FIG. 1. For example, a typical glass substrate is of the order of a millimeter thick and a typical electrochromic device covers the fully exposed area of the architectural glass, or rear-view mirror, for example. Other substrate materials may be used, for example plastics such as polyimide (PI), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN). Typical component layer thicknesses are given in the table below:
Component LayerThickness (microns)lower TCO layer0.1 to 1.0cathode0.1 to 1.0solid electrolyte0.005 to 0.5 counter electrode0.1 to 1.0upper TCO layer0.1 to 1.0diffusion barrier layer0.1 to 1.0
Switching from a transparent to a colored state, for example, occurs when ions (such as lithium or hydrogen ions) are driven from the counter electrode 150, through the (non electrically conductive) solid electrolyte 140, to the cathode 130. The counter electrode 150 is an ion storage film, and the cathode 130 is electrochromic—providing the desired change in light transmission properties. It is also possible for the counter electrode 150 to function as the electrochromic layer if this layer undergoes an “anodic coloration,” where the layer changes from transparent to colored with de-intercalation of the ion. In this case, the cathode becomes the counter electrode. One can also create greater contrast by combining the effects of both electrodes. A more detailed discussion of the functioning of electrochromic devices is found in Granquvist, C. -G., Nature Materials, v5, n2, February 2006, p 89-90. For the device to function properly, the lower TCO layer 120 and the cathode 130 must be electrically isolated from the counter electrode 150 and upper TCO layer 160. Electrical contact to external driver circuits is made through the first and second electrical contacts 180 and 190.
As is clear from FIG. 1, the device 100 requires patterning of the five active device layers 120-160. This patterning can be done using conventional physical/shadow mask-based lithography techniques. The use of traditional lithography techniques with physical masks leads to many disadvantages, especially related to high volume manufacturing (HVM). For example, the use of physical masks: (1) adds a significant capital investment requirement for HVM and large area scaling; (2) increases the cost of ownership (consumable mask cost, cleaning, chemicals, etc.); (3) decreases the throughput because of alignment requirements; and (4) results in a yield loss, since the masks are prone to flaws which translate to defects in the electrochromic devices, such as color centers and pinholes in protective layers. The presence of pinholes in protective layers will eventually lead to failure of the electrochromic devices due to oxidants reaching the active layers of the devices. This occurs for electrochromic devices sealed in IGUs when the IGU seal becomes compromised and atmospheric oxidants leak into the unit. The desired device lifetimes of tens of years cannot be achieved with pinhole defects in the protective layers. In HVM processes, the use of physical masks (ubiquitous for traditional and current state-of-the-art electrochromic device fabrication technologies) will contribute to higher complexity and higher cost in manufacturing. The complexity and cost result from the requirement to fabricate very accurate masks and the need to use (automated) management systems for mask alignment and regeneration. Such cost and complexity can be inferred from well known photolithography processes used in the silicon-based integrated circuit industry. In addition, the higher cost results from the need for maintaining the masks as well as from throughput limitations by the added alignment steps. The adaptation becomes increasingly more difficult and costly as the manufacturing is scaled to larger area substrates. Moreover, the scaling (to larger substrates) itself can be limited because of the limited availability and capability of the physical masks. This is particularly critical for the architectural window applications, where innumerable shapes and sizes are required. Therefore, there is a need for cost effective, flexible and high volume manufacturing compatible fabrication methods for electrochromic devices. Furthermore, due to the yield issues associated with mask-based lithography fabrication steps, there remains a need for improved methods for patterning the numerous component layers of electrochromic devices.
Patterning of the five active device layers 120-160, shown in FIG. 1, can be done using laser scribing techniques. See U.S. Pat. No. 5,724,175 to Hichwa et al. However, the laser scribing method of Hichwa et al. results in contamination of the exposed edges of the active electrochromic layers due to redeposition during laser ablation of material on the walls of the trench being cut. This contamination can impair the performance of the electrochromic device. Furthermore, particulates are generated during laser ablation and these particles are deposited on the surface of the device. When the protective coating is applied, the presence of particulates on the surface can lead to pinholes in the coating. Pinholes in the protective coating can result in exposure of the device to oxidants from the ambient, and ultimately to premature device failure. There is a need for laser scribing processes which do not impair electrochromic device performance.
In conclusion, there is a need for improved patterning processes for electrochromic devices and a need for improvement in the integration of patterning processes into device fabrication for electrochromic devices.